Usually, in the hydroforming process, the working fluid is introduced inside the metal tube as of the starting material (hereinafter, referred to as “metal tube blank(s)”) and the pressure is applied (hereinafter, referred to as “internal pressure”) to bulge-form said blank in the manner of following the contour of a mold that holds said blank, whereby the tubular part with a complex shape can be fabricated. Thus, the hydroforming process is widely applied for processing automobile parts.
These automobile parts generally require various piercing operations such as the one for the holes for use in fixing other parts or the other for the holes for use in a locator, so that after forming into a predetermined shape, the piercing operation is occasionally required. In this occasion, when the piercing operation becomes necessary for the press stamped parts of steel sheets, the dies and punch can be employed as a tool and holes are made at the predetermined positions where appropriate.
For all this, as the part formed by the hydroforming process entails the tubular shape, it is difficult to arrange dies at the predetermined positions inside the tube, except proximity to tube ends. Therefore, it is not possible to readily conduct a piercing operation using the punch and dies in the similar way to the piercing for the press stamped parts of steel sheets.
Accordingly, there has been proposed various methods utilizing the internal pressure to perform a piercing operation onto the hydroformed parts so far. For instance, in Japanese Patent Application Publication No. 6-292929 (sections [0036] and [0037], FIGS. 21 and 22), there is proposed a method for punching the holes wherein for the tubular frame member, holes are made by punching shortly after completing the formation of the bulge-formed portion by the hydroforming, in the state that high internal pressure is kept (hereinafter, referred to as “a first prior art”).
And in the Japanese Patent Application Publication No. 2001-18016, there is disclosed a piercing method that: the die port is disposed onto the mold; a punch is inserted so as for its top face to become flush with the inside contour surface of the mold cavity to bulge-form the metal tube blank; the punch is retreated while the internal pressure is maintained; the internal pressure is applied towards the bulge-formed portion formed at the bottom of the die port; and then, a piercing operation is performed (hereinafter, referred to as “a second prior art”).
According to the proposed “a first and second prior arts”, since after bulge-forming by the hydroforming, the internal pressure is applied and the piercing is performed onto the bulge-formed portion, the bulging-forming and piercing steps can be adapted within the series of processing steps, whereby a preconceived effects can be expected in view of the fabrication costs and operability. That said, these expected effects are limited to the case where the holes have the circle or near-circle configuration in piercing.